Which one of the following pairs is isostructural (i.e., having the same shape and hybridization)?

To determine if species are isostructural, we calculate their steric numbers and hybridization based on VSEPR theory:

• $\text{BCl}_3$ and $\text{BrCl}_3$: $\text{BCl}_3$ has 3 bond pairs and 0 lone pairs ($sp^2$ hybridization, trigonal planar). $\text{BrCl}_3$ has 3 bond pairs and 2 lone pairs ($sp^3d$ hybridization, T-shaped). They are not isostructural.
• $\text{NH}_3$ and $\text{NO}_3^-$: $\text{NH}_3$ has 3 bond pairs and 1 lone pair ($sp^3$ hybridization, trigonal pyramidal). $\text{NO}_3^-$ has 3 bond domains and 0 lone pairs ($sp^2$ hybridization, trigonal planar). They are not isostructural.
• $\text{NF}_3$ and $\text{BF}_3$: $\text{NF}_3$ is $sp^3$ hybridized (trigonal pyramidal), whereas $\text{BF}_3$ is $sp^2$ hybridized (trigonal planar). They are not isostructural.
• $\text{BF}_4^-$ and $\text{NH}_4^+$: In $\text{BF}_4^-$, the central boron atom has 4 valence electrons (3 + 1 from the negative charge) forming 4 bond pairs and 0 lone pairs ($sp^3$ hybridized, tetrahedral). In $\text{NH}_4^+$, the central nitrogen atom has 4 valence electrons (5 - 1 for the positive charge) forming 4 bond pairs and 0 lone pairs ($sp^3$ hybridized, tetrahedral). Since both have the same $sp^3$ hybridization and tetrahedral shape, they are isostructural.